This invention relates to magneto sensitive fluid composition exhibiting electrical switching as well as magnetorheological characteristics in the presence of external magnetic field and a process for preparing the same.
Ferrofluids are colloidal liquids in which ferromagnetic materials are uniformly suspended and which exhibit changes in their rheological characteristics in the presence of external magnetic field. These ferrofluids could be electrically non-conductive as well as electrically conductive. Electrically conductive ferrofluids comprise a liquid carrier medium, finely divided magnetic particles and electrically conductive particles to impart electrical conductivity to the ferrofluid. The carrier fluids employed in the ferrofluids could be hydrocarbons, mineral oils, ester based oils or even water. The magnetic particles employed in the ferrofluids could be ferromagnetic materials such as nickel, cobalt, iron, metal carbides, metal oxides and metal alloys etc. Generally, the size of ferromagnetic particles is less than 1000 angstroms. To impart conductivity to the ferrofluids, various forms of carbon like graphite, diamond etc. are used. The magnetic particles and electrically conductive particles are uniformly dispersed and stabilised by using surfactants. Again, a variety of surfactants are utilised depending upon the need for dispersion and uniformity. These non-conducting as well as conducting ferrofluids are known in the prior art. However, these ferromagnetic compositions do not exhibit significant change in their conductivity in presence of any external magnetic field.
A magnetorheological fluid composition comprises magnetic sensitive particles dispersed in a carrier fluid with the help of surfactants. The magnetic responsive particles employed could be iron oxide, iron, iron carbide, low carbon steel or alloys of cobalt, zinc, nickel, manganese etc. The carrier fluid employed could be mineral oils, hydrocarbon oils, polyester and phosphate esters etc. These magnetorheological fluid compositions exhibit changes in its rheological characteristics when subjected to external magnetic field. In absence of magnetic field, the magnetorheological fluids have measurable viscosity, which depends upon several parameters like shear rate, temperature etc. However, in the presence of an external magnetic field, the viscosity of the fluid increases to a very high value as the suspended particle align themselves resulting in rapid physical gelling of the fluid. These known magnetorheological fluids are either electrically insulating or conducting. Although, a few magneto active materials exhibit change in electrical conductivity in the presence of an external magnetic field, these materials are neither fluids nor they exhibit any significant change in, their electrical conductivity.
These magnetorheological as well as ferromagnetic fluid compositions, known in the prior art suffer from following disadvantages.
The main disadvantage of magnetorheological as well as ferromagnetic fluids known in the prior art is that these fluid compositions do not exhibit any significant change in electrical conductivity under the influence of external magnetic field and as such these fluids can not be utilised in electrical switching applications.
Another disadvantage of magnetorheological as well as ferromagnetic fluids known in the prior art is that these fluid compositions do not exhibit any change in capacitance value under the influence of external magnetic field and as such these fluid can not be utilised for such an applications where variation in capacitance is required.
Primary object of the invention is to provide a magneto sensitive fluid composition and a process for preparing the same wherein the composition exhibits excellent electrical switching characteristics, in addition to magnetorheological characteristics, in the presence of an external magnetic field.
Another object of the invention is to provide a magneto sensitive fluid composition and a process for preparing the same wherein the electrical resistance of the composition can be continuously varied from a high value of 10 ohm to a very low value of 1 ohm depending upon the strength of the external magnetic field applied.
Still another object of the invention is to provide a magneto sensitive fluid composition and a process for preparing the same wherein the composition exhibits change in capacitance over a wide range under the influence of an external magnetic field.
Yet further object of the invention is to provide a magneto sensitive fluid composition and a process for preparing the same wherein the composition has excellent magnetorheological properties in combination with electrical switching characteristics.
Still another object of the invention is to provide a magneto sensitive fluid composition and a process for preparing the same wherein the composition has excellent magnetorheological properties in combination with variable capacitance.
Still further object of the invention is to provide a magneto sensitive fluid composition and a process for preparing the same wherein the composition has low hysteresis characteristics.
Yet further object of the invention is to provide a magneto sensitive fluid composition and a process for preparing the same wherein the composition can be used over a wide operating temperature range varying from xe2x88x9210xc2x0 C. to +80xc2x0 C.
Still another object of the invention is to provide a magneto sensitive fluid composition and a process for preparing the same wherein the viscosity of the composition along with electrical resistance and capacitance can be continuously varied by varying the strength of the external magnetic field.
Yet further object of the invention is to provide a magneto sensitive fluid composition and a process for preparing the same wherein the Brookfield Viscosity of the composition can be changed continuously over a wide range, typically from 700 CP to 120000 CP or better.
Still further object of the invention is to provide a magneto sensitive fluid composition having variable electrical resistance and capacitance for making sensors or devices wherein change of either electrical resistance or capacitance in the presence of a magnetic field is desired. Few examples of such possible sensors or devices are non-arcing relays, high voltage protector, variable resistors, tilt sensors, magnetic mine sensors, microwave shielding devices, proximity fuses for torpedoes etc.
According to this invention there is provided a magneto sensitive fluid composition having electrical switching and magnetorheological characteristics in presence of an external magnetic field, comprising:
a) a carrier fluid,
b) magnetic sensitive particles comprising 85-98% by weight of high purity iron particles, such as carbonyl iron, dry blended with 2-15% by weight of ferrite,
c) magnetic sensitive particles comprising 50-90% by weight of said magnetic sensitive particles admixed with 10-50% by weight of a conductive metallic or non-metallic additive,
d) magnetic sensitive particles stabiliser synthesised from said carrier fluid; said doped magnetic sensitive particles coated with said magnetic sensitive particles stabiliser uniformly dispersed in the said carrier fluid.
The external magnetic field, induces alignment in the magnetic sensitive particles dispersed in the carrier fluid medium which, in turn, apart from changing the reheological characteristics, also changes the electrical conductivity of the composition. Apparently, the aligned magnetic sensitive particle act in an organised manner so as to facilitate conduction of electrons induced by the added additives. This conduction of electrons is essentially responsible for change in the characteristic of the fluid from a non-conducting material to a conductive material. The suspended particles align to form a chain like structure in the presence of a magnetic field and a conductive path is formed for the conduction of electrons. Through this path, the electrons contributed by the added additives conduct and fluid starts behaving as a conductive material. Once the external magnetic field is removed, the alignment of magnetic particles is disturbed and the conduction path for the electrons is no longer available. This results in the reversal of the characteristic of the material and it starts behaving as an insulator.
The present composition utilises a derivative of vegetable oil extracted from agro-seed such as castor oil as carrier fluid. This carrier fluid i.e. vegetable oil is cheaper, easily available, eco-friendly, bio-compatible and has a renewable source of supply. The composition utilises iron and its alloys, iron oxides, iron carbide, carbonyl, iron nitrides etc. as magnetic sensitive particles. The proposed process for preparation of the magneto sensitive fluid composition is simpler and it does not require complex machinery. Further, the composition is highly homogeneous as it utilises magnetic sensitive particles modifier or surfactant, which is synthesised from the very carrier fluid, employed in the composition. This surfactant improves the homogeneity of the composition and reduces gravity settling problems of the magnetic sensitive particles.
The useful conductive metal additives include powders of gold, silver, copper, aluminum, or any other conductive metallic powders, while conductive nonmetallic powders include graphite, conductive carbon black or any other nonmetallic conductive powders.
The present magneto sensitive composition can be utilised for making sensors or devices wherein change of either electrical resistance or capacitance in the presence of a magnetic field is desired. Few examples of such possible sensors or devices are non-arcing relays, high voltage protector, variable resistors, tilt sensors, magnetic mine sensors, microwave shielding devices, proximity fuses for torpedoes etc.
(i) Preparation of Magnetic Sensitive Particle
85-98% by weight of high purity iron particles (such as carbonyl iron) and 2-15% by weight of ferrite of nickel and zinc (such as nickel-Zinc ferrite) are dry blended using a powder blender.
(ii) Admixing of the Magnetic Sensitive Particles with Conductive Particles
50-90% by weight of the mix obtained from step (i) is dry blended with 10-50% by weight of any conductive metallic or non metallic powder such as silver, graphite powder etc. using a powder blender.
(iii) Preparation of Stabiliser for Magnetic Sensitive Particles Obtained from Step (ii)
0.50-2.5% by weight of con, sulphuric acid (assay 98%) is poured drop wise to 95-99% by weight of a carrier fluid preferably commercially available castor oil (viscosity about 700-800 Cps,) and mixed using a laboratory stirrer at a temperature between 25-30xc2x0. The mix is allowed to react for two hours while maintaining the temperature between 25-30xc2x0.
To the above mix, 0.5-2.5% by weight of 20% aqueous solution of potassium hydroxide (potassium hydroxide pellets  greater than 85% purity dissolved in distilled water) is added drop wise and mixed using a laboratory stirrer. The reaction is allowed to continue for about two more hours. The temperature, throughout the reaction, is maintained between 25-30xc2x0 C. by using a water bath. The particle stabiliser, thus obtained, is washed with distilled water till the pH of water becomes neutral.
(iv) Coating of Magnetic Sensitive Particles Obtained from Step (ii) with the Stabiliser Obtained Form Step (iii)
1-10% of the magnetic particles stabiliser obtained from step (iii) is preheated to a temperature between 60 and 80xc2x0 C. and poured drop wise to 90-99% by weight of the magnetic sensitive particles obtained from step (ii) in a laboratory kneader and is mixed properly. The stabiliser coated magnetic sensitive particles, thus obtained are in the consistency of putty. This putty is allowed to mature for 24 hours at a temperature between 25-30xc2x0 C.
(v) Synthesis of Magneto Sensitive Fluid Formulation
80-90% by weight of coated and magnetic sensitive particles obtained from step (iv) is mixed with 10-20% by weight of the carrier fluid as used in step (iii) preferably commercially available castor oil (viscosity between 500-700 Cps). Before mixing, the carrier fluid, preferably commercially available castor oil, is heated up to 60-80xc2x0 C. in a container and the said coated and magnetic sensitive particles are added to it in a gradual manner under continuous stirring with a laboratory stirrer. The entire mix is further homogenised in a high-speed mixer by raising the mixing speed from low rpm to about 2000 rpm within the first 10 minutes of mixing. The mixing is continued for about one hour at this mixing speed followed by cooling of the mix to about 30xc2x0 C. The mixture is further agitated at a high rpm of about 2500-3000 for about 3-5 minutes and followed by cooling it to room temperature. The above agitation process at 2500-3000 rpm is repeated once again to finally obtain magneto sensitive fluid composition.
The invention will now be illustrated with working examples, which are typical examples to illustrate the working of the invention and are not intended to be taken restrictively to imply any limitation on the scope of the present invention.